As shown in FIG. 1, a computed tomography (CT) scanner for producing images of the human anatomy includes a motorized patient table 10 which positions a patient at different depths within aperture 11 of a gantry 12. A source of collimated X-rays 13 is mounted within the gantry 12 to one side of its aperture 11, and an array of X-ray detectors 14 is mounted to the other side of the aperture. The X-ray source 13 and detectors 14 revolve about aperture 11 during a scan of the patient to obtain X-ray attenuation measurements from many different angles. U.S. Pat. Nos. 4,112,303 and 4,115,965 disclose the details of the gantry construction, and U.S. Pat. No. 4,707,607 describes the detector array 14. The descriptions of the components in these patents are incorporated herein by reference.
A complete scan of the patient is comprised of a set of X-ray attenuation measurements which are made at different angular orientations of the X-ray source 13 and detector 14 in one revolution about the patient. The gantry may stop or continue to move as the measurements are being made. An attenuation measurement at a given orientation is referred to in the art as a "view" and the set of measurements at a view forms a "transmission profile." As shown in FIG. 2, the X-ray source 13 produces a fan-shaped beam that passes through the patient and impinges on an array of detectors 14. Each detector 14 in this array produces a separate attenuation signal and the signals from all the detectors 14 are separately acquired to produce the transmission profile for the indicated angular orientation. The profiles are stored in a disc memory as raw, or uncompensated, data. The X-ray source 13 and detector array 14 continue to revolve in direction 15 to a another angular orientation where the next transmission profile is acquired.
The resultant transmission profiles from the scan then are used to reconstruct an image which reveals the anatomical structures in a slice taken through the patient. The prevailing method for reconstructing image is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Each X-ray detector 14 comprises a scintillator and a solid state photodiode. X-rays striking the scintillator produce light photons which are absorbed by the photodiode creating an electric current. The light is not emitted by the scintillators instantaneously, rather the emission follows a multi-exponential curve. Similarly, the light emission does not terminate immediately when the X-ray beam is extinguished, but produces a response from the detector having a decay which can be defined by a multi-exponential function. The time dependence of the output signal intensity can be modelled accurately as a sum of several different time constant decay components.
Because the detector array is rotating rapidly about the patient, the exponential decay blurs together detector readings for successive views creating an adverse effect that is referred to as "afterglow". The afterglow is a function of the intensity of the X-ray flux and the response characteristics of the detector, and degrades azimuthal resolution of the image and produces shading and arc shaped artifacts. The azimuthal direction 16 of the image area is transverse to a line 17 radiating from the center of the imaging aperture 11. The amount of blurring increases the farther the object is spaced from the aperture center, since the speed at which the object is swept across the detectors 14 effectively increases with this spacing.
FIG. 3 plots attenuation values from a given detector for a series of views and graphically depicts the blurring. The solid line represents the output of a single detector 14 during several views for a square object being imaged. Ideally the detector data should have a rectangular shape as represented by the dashed lines. However, the effect of the afterglow blurring rounds the edges of the waveform and extends the object signal into several adjacent views. When the views are used to reconstruct an image, the object will appear enlarged and will not have sharp, distinct edges.
The CT system may be operated in either the axial mode, the helical scan mode or the cine mode. In the axial mode, the table 10 is stationary during each scan and the gantry revolves once to complete a scan. A compensation technique for the afterglow artifacts in the axial mode has been proposed in U.S. Pat. No. 5,249,123 entitled "COMPENSATION OF COMPUTED TOMOGRAPHY DATA FOR DETECTOR AFTERGLOW ARTIFACTS". This method uses the detector output sample from the previous view to derive a compensation value for each detector response decay component. The sample for the present view is adjusted according to the compensation values to remove the afterglow artifacts. The compensation values for the first view of the scan are zeroes, since a relatively long time occurs between scans allowing the afterglow to decay to a negligible level.
In the helical scan mode, the gantry revolves through many revolutions and the table moves the patient through the aperture 11 during the examination. A helical scan is subdivided to a series of individual scans, each comprising one revolution of the gantry and the data acquired during each individual scan is stored in a separate file in the CT system. In the helical mode, time between each individual scan is equivalent to the time between each view within a scan and the afterglow no longer decays to a negligible level before the next scan starts. Therefore, the first view of each scan in the helical mode will be affected by the afterglow from the last view of the previous scan.
As the data for the helical scan is stored as separate files for each individual scan, attenuation data for the previous scan is not available for deriving compensation values for the afterglow in the next scan in the helix. Merely setting the compensation values for the first view in each scan to zero does not eliminate the afterglow artifacts. This problem also occurs in the cine mode in which the table remains stationary while the gantry moves continuously through many revolutions acquiring a series of individual scans. Therefore, another technique must be employed to compensate for afterglow in the first view of successive individual scans in the helical and cine scan modes.
Further, the image data are archived as raw, uncompensated, attenuation measurement values and a similar problem exists during subsequent reconstruction of an image from the archived data. A technician is able to specify any location in the helical scan data at which to begin image reconstruction and the reconstruction process uses one revolution worth of views from that point. Whenever a location other than the first acquired view is specified, the computer processor will lack information about the previous views from which to compute afterglow compensation values for the first view of the reconstruction.